Board on chip (BOC) module arrangements are produced by a mounting process in which bond pads (contact areas) have first been provided on the active side of a chip either directly with solder balls or with other 3-dimensional (3D) structures from a solder material, or structures containing at least one solder material or the like, for example by printing or dispensing. The chip is subsequently positioned face down over the terminal contacts on a printed circuit board (PCB) and then attached under the effect of heat by soldering. In this operation, an electrical and at the same time mechanical connection takes place between the chip and the printed circuit board.
In the case of such BOC module arrangements it has proven to be disadvantageous, however, that considerable mechanical stresses may occur between the different materials of the chip and the printed circuit board during normal operation caused by the different coefficients of expansion of the chip and of the printed circuit board. This thermal mismatch may lead in long-term operation to the failure of the BOC module arrangement, in that for example one or more of the solder connections rupture due to mechanical overload. The consequence is then generally total failure of the component.
To eliminate or at least minimize such stress caused by a thermal mismatch, it has been the practice to use an underfiller with good flow properties, introduced between the chip and the printed circuit board by capillary action after the chip has been mounted on the printed circuit board.
The particular disadvantage of using such an underfiller is that the mounting of the chip on the printed circuit board has to be performed first, and only then can the underfiller be introduced. This means that each individual chip has to be stabilized by the underfiller after it has been mounted. This also has the result that the introduction of the underfiller is a very laborious procedure because every BOC arrangement has to be individually enhanced. It is also difficult to introduce the underfiller uniformly.
The problems of the mechanical strength of the connection between the chip and the printed circuit board are exacerbated if, instead of the relatively solid solder balls of a solder material, so-called soft solder balls of a mixture of an organic material and a solder material, or even 3-dimensional compliant elements are used as electrical and mechanical connecting elements. Several problems have to be simultaneously taken into consideration here.
The soft solder balls have by their nature a lower mechanical strength, so that the connection between the chip and the printed circuit board is less strong for this reason alone. It is even more problematic when compliant 3D elements are used. Here, a subsequent increase of the mechanical strength of the connection between the chip and the printed circuit board is particularly important because the compliant 3D elements consist of a compliant base element, on which an electrically conductive structure of metal is then applied. Here, the electrically conductive structure (reroute layer/wiring) cannot transfer any mechanical forces at all.